1. Field of the Invention
This invention relates to narrow-band, relatively ultra-stable radio apparatus for communicating signals from protected premises or other locations from which messages must be transmitted to a central monitoring point, such as a police station or maintenance center.
2. Description of Prior Art
The transmission of priority signals such as burglary, fire, emergency medical, and other signals represents an important segment of the communication art wherein high reliability and immediacy of transmission are important. The alarm industry has customarily employed telephone lines for this purpose and in particular has used a so-called DC circuit wherein a continuous direct current path is established between a sender and a receiver to provide a channel of communication and to detect the presence of tampering or interference, for example the cutting of a telephone line. Such DC telephone paths are becoming obsolete and are being replaced by more expensive multiplexed telephone circuits employing subcarriers. A strong need has arisen for alternative means of alarm communication.
Radio alarm communication using conventional techniques has not been very reliable because of the relatively poor signal-to-noise ratio that results from interference from both natural and man-made signals and because of interference from legitimate signals sharing the channel which may be using the channel when an alarm condition occurs. The scarcity of radio-frequency channels further aggravates this condition.
Radio alarm transmitters in the prior art generally incorporate both a receiver and a transmitter so that they may monitor for the presence of radio transmission on their channel prior to transmitting an alarm signal in order to avoid interference or masking of signals which could negate both the alarm transmission and the intelligibility of the interfering signal.
Prior-art devices have not been able to economically employ narrow-band transmission techniques because the available frequency determining sources, for example crystal oscillators, have not been sufficiently stable to permit very narrow-band transmissions that are commensurate with the bandwidth of the alarm and status signal information content (e.g. under 100 Hz bandwidth) and acceptable transmission time. Prior-art devices are also relatively complicated when designed to use synchronous detection techniques in the receiving circuits since no local frequency reference for the synchronous process is available and it must be generated from the usually weak incoming alarm signal.
Swanson (U.S. Pat. No. 3,883,874) has disclosed an apparatus which provides a source of standard frequency for radio communication and navigation. This apparatus is phase locked to commuted signals at the same frequency using a multiplexed antenna. Swanson employs very-low frequency (VLF) radio signal sources, specifically highly stable OMEGA navigation signals, or possible other highly stable VLF transmissions such as standard radio transmission from the National Bureau of Standard WWV stations, in order to generate absolute standard frequencies. The detection and use of such standard frequency transmissions is unnecessarily complicated because special radio receiver circuits are needed to detect VLF signals and because of the attempt to generate absolute standard frequency.